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Ministry of Agriculture, Food and Rural Affairs

Black Knot

Black knot parasitized by Trichothecium Black knot with peach tree borer frass Black knot on second year wood Black knot on tart cherry Black knot on tart cherry
Click to enlarge.

Beginner

This pest affects:

Peach Apricot Plum Sweet Cherry Tart Cherry  

Scientific Name
Apiosporina morbosa (Dibotryon morbosum)

Identification
Branches, shoots and twigs
Late summer of year of infection (plum only) or spring of the following year (plum & sour cherry)

  • Light brown swellings about 1 cm long on one side of the current shoots
  • Occasionally, a few small knots on plum twigs will develop in to mature black knots during the same season of infection

Second growing season after infection

  • Knots longer and olive-green colour and with a velvety texture eventually turning black and knots soon grow darker

Fall of the second year after infection

  • Knots coal-black and woody, 10 to 15 cm long and may completely encircle the limb
  • Knots 30 cm or more long may result from the fusion of two or more knots
  • May become covered with pinkish-yellow parasitic fungus, Trichothecium roseum

Subsequent years

  • Knots continue to expand in following the branch becomes girdled and dies
  • Older knots are frequently invaded by wood-boring insects
  • Severely diseased trees have fewer blooms, poor fruit production and are more susceptibility to winter injury
  • Heavily infected trees become unproductive after a few years due to the loss of vigour and limb death caused by the disease

Blossoms, leaves, and fruit tissue are not infected by A. morbosa.

Often Confused With
Canker – gummy, sunken areas; no black swollen growth

Period of Activity
Symptoms of black knot first appear as light brown, swellings about 1 cm long on the shoots of the current season's growth in late summer or the spring following infection. On some plum trees the swellings show up in August or September (3-4 months after the initial infection) while in sour cherries and some plums the disease symptoms may not be expressed until the following March or April (11-12 months after the initial infection).

Scouting Notes
During the dormant period, monitor in a block that includes a susceptible cultivar, such as 'Stanley'. Presence of any black knots represents high risk on susceptible cultivars. Awareness of black knot inoculum from adjacent commercial orchards or wild hosts may affect control decisions.

Continue to monitor for black knots during the bud-break to bloom period.

Threshold
There is no threshold established.  However, it is critical to removal all knots to prevent spread of the disease in the orchard as a single knot can produce thousands of spores.

Advanced

This pest affects:

 

Scientific Name
Apiosporina morbosa (Dibotryon morbosum)

Identification
Branches, shoots and twigs
Late summer of year of infection (plum only) or spring of the following year (plum & sour cherry)

  • Light brown swellings about 1 cm long on one side of the current shoots
  • Occasionally, a few small knots on plum twigs will develop in to mature black knots during the same season of infection

Second growing season after infection

  • Knots longer and olive-green colour and with a velvety texture eventually turning black and knots soon grow darker

Fall of the second year after infection

  • Knots coal-black and woody, 10 to 15 cm long and may completely encircle the limb
  • Knots 30 cm or more long may result from the fusion of two or more knots
  • May become covered with pinkish-yellow parasitic fungus, Trichothecium roseum

Subsequent years

  • Knots continue to expand in following the branch becomes girdled and dies
  • Older knots are frequently invaded by wood-boring insects
  • Severely diseased trees have fewer blooms, poor fruit production and are more susceptibility to winter injury
  • Heavily infected trees become unproductive after a few years due to the loss of vigour and limb death caused by the disease

Blossoms, leaves, and fruit tissue are not infected by A. morbosa.

Often Confused With
Canker – gummy, sunken areas; no black swollen growth

Biology
Black knot can infect peach, nectarine, sweet cherry and apricot but is most frequently found on plum and sour cherry.  Wild plum, chokecherry and ornamental Prunus trees are also hosts and can be a source of inoculum to nearby commercial orchards. The black knots tend to be largest on the very susceptible and moderately susceptible varieties of European plum and smaller on less susceptible Japanese plums. The strain of A. morbosa that causes small knots on sour cherry will not infect plum and plum strains will not infect cherry.

The disease cycle of black knot (the time from initial infection until a black knot releases spores) normally takes 2 years to complete.

The fungus overwinters in knots or in the infected wood around the knots. Fruiting bodies are produced on the outside of the knots either during the fall or over the winter. In the spring, the fruiting bodies begin to produce spores (ascospores). The development of ascospores is temperature-dependent: in years with a cold spring, ascospore development may be delayed. Ascospores are ejected from the knots during rainy periods from the end of March through the middle of July. The peak period for spore release usually occurs from late May (shuck split) through the end of June. However, in a warm, dry spring, spores develop and mature but are not released until rainfall occurs.

At least 2 mm of rain are required for spore release from knots on sour cherry and spores can continue to be released for up to 3 days after the rain has stopped. They are spread by wind and rain to twigs where infection takes place through unwounded tissue. Infections can occur on developing shoots when temperatures reach 10-11°C and a wetting period occurs for at least 6 hours although optimum conditions for infection are 20°C with wetness periods of 48 hours following rain. 

Very young, succulent shoots of the current year’s growth are susceptible to infection from bud break through terminal bud set particularly during cool, wet spring weather. Infections will often occur in the axil of leaves where rainwater collects and tissue remains wet for longer periods. The fungus will grow from infected developing fruit spurs into older limbs. Knots often start to form near the point of leaf attachment.  They are initially green and soft, later turning dark and woody.  As it colonizes the woody tissue of infected shoots and limbs it stimulates the surrounding tissue to produce tumor-like corky brown growth. First year knots (12 months after infection) produce summer spores that do not cause disease. The fungus extends several inches beyond the knots and knots will expand with age.

The following spring, the brown corky swellings continue to develop and expand, transforming into mature black knots by autumn. The fungus overwinters as a mature black knot during the second year and produces spores for release early the following spring. Black knots will continue to expand over time and eventually surround the twig or branch.

Period of Activity
Symptoms of black knot first appear as light brown, swellings about 1 cm long on the shoots of the current season's growth in late summer or the spring following infection. On some plum trees the swellings show up in August or September (3-4 months after the initial infection) while in sour cherries and some plums the disease symptoms may not be expressed until the following March or April (11-12 months after the initial infection).

Scouting Notes
During the dormant period, monitor in a block that includes a susceptible cultivar, such as 'Stanley'. Presence of any black knots represents high risk on susceptible cultivars. Awareness of black knot inoculum from adjacent commercial orchards or wild hosts may affect control decisions.

Continue to monitor for black knots during the bud-break to bloom period.

Threshold
There is no threshold established.  However, it is critical to removal all knots to prevent spread of the disease in the orchard as a single knot can produce thousands of spores.

Management Notes
Clean up trees annually. Prune out infected branches well before bloom, preferable in late winter before new growth begins.  Remove infected branches to at least 15-20 cm below knot. NOTE: It is preferable to prune an infected branch further back to an appropriate location, such as a healthy collar, rather than leave a stub. Remove pruned knots from the orchard and promptly burn or bury them beneath at least 30 cm of well-packed soil. Chopping prunings with a flail mower (to strip infected bark and knots from wood) is an alternative method of destroying infected prunings if burning or burying is impractical. Pruned knots left on the orchard floor can produce and release spores. Eliminating all sources of inoculum from the orchard is vital to prevent further infections.

Chisel out black knots on major scaffolds or trunks by removing at least 10 cm of tissue beyond visible knots because the fungus colonizes the inner bark beyond the visible swelling. Failure to remove enough tissue can result in the regrowth of the knot.

Do not plant new plum or tart cherry orchards next to trees or orchards where black knots are present.

If possible, remove wild plums and cherries from fencerows and wooded areas within 150 m around orchards to reduce the potential of spores from knots on wild hosts blowing into orchards.

Purchase and plant disease-free trees from reputable nurseries. Never buy or plant any nursery stock that has visible knots or abnormal swellings on twigs and branches.

Cultivated plums vary greatly in their susceptibility to black knot.

Suceptibility of Plum varieties to Black Knot
(E) European plum  (J) Japanese plum


Very Susceptible

Moderately Susceptible

Slightly Susceptible

Bluefre (E)

Early Italian (E)

VioletteTM (E)

Damson (E)

Italian (E)

Burbank (J)

Stanley (E)

Valor (E)

Early Golden (J)

Veeblue (E)

Voyageur (E)

Shiro (J)

Vision (E)

Valerie (E)

 

Ozark Premium (J)

Vanette (E)

 

 

VibrantTM (E)

 

 

Vanier (J)

 

Fungicides will not provide adequate control of black knot without proper orchard sanitation (pruning, removal, and burning of black knots). Many fungicides registered for brown rot control can help with black knot management, but only by suppressing the black knot disease. Most fungicides targeted for brown rot control will prevent 40%-50% of the potential black knot infections under high disease pressure. Where cultural controls such as pruning knots are combined with the use of fungicides, up to 90% prevention of black knot.

During warm, dry springs, it may be necessary to maintain protective fungicide coverage for a longer period to avoid later infections.

Management with fungicides – Fungicides are used to control black knot in commercial orchards.  See OMAFRA Publication 360, Guide to Fruit Production.

Chapter 6 Plum Calendar: Recommendations for Black Knot at Popcorn, Shuck fall, First cover.
Chapter 6 Sour Cherry Calendar: Recommendations for Black Knot at Bloom, Petal fall, Shuck split.

Some of the information included above has been excerpted from : http://www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1198101468695&lang=eng

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/faq7622

http://nysipm.cornell.edu/factsheets/treefruit/diseases/bk/bk.asp